Semiconductor devices, such as integrated circuits and semiconductor chips, are usually encapsulated in a protective package made of plastic, metal or ceramic material. The package provides hermetic sealing and structural rigidity to the device. The package has a plurality of conductive leads extending therefrom for attachment to electrical conductors on printed circuit boards. The leads are usually made of a metal alloy and may be soldered directly on the surface of the conductor, i.e., "surface mounted" on the printed circuit board. Surface mounting of leads eliminates the need for drilling holes in the printed circuit board, and extending leads through the holes, and soldering the leads in the holes of the board. More importantly, with surface mounting, the package leads can be positioned closer together to achieve a higher component density on the printed circuit board.
To further increase component density on a printed circuit board, thin packages that are vertically mounted are utilized. Thin packages with mounting studs and leads extending from one side of the package are vertically mounted on a printed circuit board using the mounting studs. Once mounted, the leads of the thin package are surface mounted on the printed circuit board. Vertical mounting of a package will minimize the space requirement of the package on a printed circuit board and thereby increase the density of components on a printed circuit board.
A typical vertically mounted thin package has one row of leads extending from one side of the package for surface mounting on a printed circuit board. Certain applications, however, may require thin packages to have two rows of leads. For example, as disclosed in concurrently filed U.S. patent application Ser. No. 08/134,035, a high density semiconductor package encapsulating two semiconductor chips may require two rows of leads, i.e., one row of leads for each chip. In another application, two rows of leads may be required for a semiconductor package housing a large semiconductor device requiring many leads. Furthermore, twin rows of leads may be used to reduce the dimensions of a package. For example, a package having one long row of leads may be reduced in size by using two shorter rows of leads.
In the prior art, because of the thinness of the package, the leads of a typical thin package with twin rows of leads would extend beyond the "footprint" of the package, i.e., the surface area of the side of the package having the leads, when used in a surface mounted application. Therefore, the leads of a prior art thin package are flared outward and away from the package, in a gull-wing shape, in order to allow for the soldering of the leads on a printed circuit board. Such gull-wing shaped leads would require more surface area on the printed circuit board, i.e., a larger "footprint," and thus reduce the component density on the board. Therefore, a need has arisen for a thin package with twin rows of leads that do not extend beyond the footprint of the package.